A description of an Acheson-type silicon carbide furnace is included in U.S. Pat. No. Re. 11,473 to Acheson re-issued Feb. 26, 1895. In the Acheson furnace, a carbon core resistor element is used and when the reaction has been completed, silicon carbide crystals surround the carbon rod in a zone that remains embedded in a more or less unreacted mix.
The Acheson furnace may be operated on a "green run" basis with a charge composed of fresh sand and coke that is heated to reaction temperatures to produce silicon carbide. During a green run about 50% of the mass is reacted to form silicon carbide crystals with impurities migrating from the reaction zone into a surrounding partially reacted mix that also contains some silicon carbide crystals, the partially reacted mix being called fire sand.
In order to avoid waste, the fire sand is mixed with additional sand and coke for making a "black run" in which the reacting mixture includes the small proportion of silicon carbide formed in the fire sand of the previous run, the impurities from that run, and the newly added sand and coke.
The unreacted and partially reacted mix left after each production run is recovered for reuse in subsequent production runs but, of course, this is wasteful in the use of energy and labor. It is heated up during the production run and then cooled down, it also requires labor for reprocessing and clean-up, and the mix then must be subjected again to a heating step in the subsequent production run.
Various attempts have been made to increase the yield of the most desired coarse crystalline silicon carbide product by increasing the size of the furnace, and increasing the power input. But such changes have not resulted in an overall increase in the efficiency of the desired silicon carbide because a larger central zone of decomposed silicon carbide material was produced and a larger band of fire sand that required recycling was inherently formed as the diameter of the reaction zone increased.
Examples of other prior art attempts to increase silicon carbide production or control the crystal growth in the resulting product are shown in U.S. Pat. No. 1,044,295 to Tone Nov. 12, 1912 wherein a zig-zag electrical resistance core structure was proposed; and U.S. Pat. No. 2,178,773 to Benner et al Nov. 7, 1939 which shows an induction heating arrangement making use of a centrally disposed susceptor to energize the reaction.